I dimly recall that if the game passes the "okay, make an artifact" check but then fails to find an artifact it can make, then it will supercharge the bonuses on the ego-item it creates instead. Maybe that's what happened here?

EDIT: with 1d3 unenchanted iron shots and that sling, you'd be doing 112 average damage per shot, 336 average per round. If the shots were +12, then you'd be doing 480 average per round instead. If they were +12 and Slay Evil, then it'd be 600/round. That's crazy.

I was using Slay Evil Mithril shot (2d4) (+14,+12) for 820 damage/turn for a good portion of the fight. I had a stack of Rounded Pebbles and Iron shot slay evil too--both of those did over 700 damage/turn. Unfortunately, there's no Holy Might mithril shot. :/

However, with normal mithril shot (+13,+14), the damage is 572/turn, so I can't complain!

I have seen holy might mithril shots but they are extremly rare. On a good run, you can get 2-3 stacks of arrows or bolts, but I have never had more than 1 stack of shots, most of the time none at all.

I have seen holy might mithril shots but they are extremly rare. On a good run, you can get 2-3 stacks of arrows or bolts, but I have never had more than 1 stack of shots, most of the time none at all.

Plus 1, here. Very rare, but when you have them and a great buckland sling, nothing compares. Morgoth is swiss cheese filled with holes and dead before he knows what happened!

__________________
“We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see.”
― Tom Stoppard, Rosencrantz and Guildenstern are Dead